An Internet protocol (IP) service can include plural items delivered using an IP session. An IP session may include an IP stream carrying primary content, such as live or recorded music, and further IP streams carrying secondary content, such as error correction or song lyrics. Another example of an IP session is news, where a primary IP stream may carry audio data, with secondary streams carrying a video clip download and a web page push respectively. Such services can be broadcast using terrestrial digital video broadcast (DVB-T), but IP streams may instead be multicast or unicast.
A DVB transmission system usually provides a data rate of 10 Mbps or more. This provides a possibility to significantly reduce the average DVB receiver power consumption by using time-slicing, which is based on time division multiplexing (TDM). Time sliced data is sent in bursts using significantly higher bandwidth than the bandwidth required if the data was transmitted using static bandwidth. Within a burst, data is included indicating the time to the beginning of the next burst (delta-t). Between the bursts, service data is not transmitted, allowing other services to use the bandwidth. This enables a receiver to stay active for only a proportion of the time, whilst still receiving bursts of data for a requested service. If the mobile handheld terminal requires a lower constant bitrate, buffering the received bursts can provide this. Data is formatted by using, for example, a multi-protocol encapsulator in accordance with Section 7 of European Standard EN 301 192 “Digital Video Broadcasting (DVB); DVB specification for data broadcasting.” The multi-protocol encapsulator sends encapsulated data to a digital broadcast transmitter for broadcast to the digital broadcast receiver as a time-sliced signal. The time-sliced signal comprises a continuous series of transmission bursts.
A DVB-T receiver terminal is able to determine information about IP sessions from IP session announcements. Announcements are structured in a hierarchal manner. At the lowest level, announcements in respect of a subcategory of information services may include a number of messages, each message having information describing the IP session, information descriptive of the content of the IP streams, information concerning the location (e.g. channel frequency, etc.) of the respective IP streams, information about schedules of sessions, and certain other parameters related to it. Above the lowest level, there are hierarchical levels of announcements, each level of which gives information only about announcements on an immediately lower level. The services typically are divided into different categories such that the highest (or root) level may include messages relating one-to-one to the categories of news, sport, entertainment, etc. Announcements in the sport category may then consist of messages each relating to a different one of football, hockey, athletics, etc. There may be any number of levels of announcement. An IP session announcement can be made using a session description protocol (SDP) message, which forms part of a session announcement protocol (SAP) message.
It is common for an IP session announcement at one level in the hierarchy to be transmitted on a channel having a different frequency to a channel on which an IP session announcement on a lower level, for example an immediately lower level, is transmitted. This means that it is not usually possible for a terminal having a single radio receiver to receive announcements on different levels simultaneously.
WO 01/99348 describes a multicast protocol.